(CNSNews.com) – With mystery continuing to surround the fate of Malaysia Airlines flight MH370, CIA Director John Brennan said Tuesday that there was “a lot of speculation right now” but he would not rule out the possibility of terrorism being a factor.

Asked during an event at the Council on Foreign Relations whether he has discounted terrorism, Brennan replied, “No, I wouldn’t rule it out, not at all.”

As an extensive search for the missing airliner continues, other theories circulating include a deliberate act on the part of a pilot, conceivably for political or personal reasons.

Citing reports that the aircraft’s transponder – a transmitter that recurrently signals an aircraft’s altitude and position – had been turned off or knocked out during the flight, Brennan said there were “a number of very curious anomalies about all of this.”

“You cannot discount any theory,” he said. “Who had the ability to turn off the transponder? I don’t know.”

“Might it have been something that had happened inside the aircraft that led to its ultimate fate? Or could it have just been some type of sort of catastrophic event that unfortunately led to that aircraft’s and those passengers’ demise? I don’t know. But I don’t think people should at this point rule out any of these lines of inquiry.”

(An EgyptAir Boeing 767 crashed into the sea off the coast of Massachusetts in 1999, killing all 217 people on board, 100 of them Americans. Exactly what happens remains unclear, but the 9/11 Commission report said “[t]he most plausible explanation that emerged was that one of the pilots had gone berserk, seized the controls, and flown the aircraft into the sea.”)

Whatever the case with the missing Malaysia Airlines Boeing 777 and its 239 passengers and crew, Brennan said terrorists remain keen to attack aircraft, and warned against complacency.

“It's close to – now 13 years since 9/11, and I think the memories and tragedy of 9/11 have receded in the minds of many people. And this is not the time to relax, because we know that there are terrorist groups that are still determined to carry out attacks, including against – especially against aircraft.”